In conventional methods for reducing ores (and for other processes for extracting ores), the heat of reaction and the reactive processing agents (generally reducing agents) are supplied by way of the gas phase. Using these methods, the fuel gases tend to contaminate the product with, for example, the sulfur and phosphorous contained in the fuel gases.
Alternately, indirectly heated rotary kilns have been used. The outer jacket of the rotary kiln is made of a highly alloyed steel containing, for example, cobalt, chromium and nickel. According to the German Offenlegungsschrift 25 26 296, such a furnace can be used for calcining at temperatures up to 1,200.degree. C.
However, the reduction of ores generally requires significantly higher temperatures range in the reducing zone of the rotary kiln. Depending on the starting materials selected and on the reactions desired, the temperatures in the reducing zone generally are between 1,000.degree. and 1,500.degree. C. However, under normal operating conditions, this temperature will be exceeded for brief periods of time. Thus, an apparatus which withstands such extreme temperatures (&gt;1,200.degree. C.) can be very useful for allowing reliable reduction processes that utilize indirect heating.
Under the temperature conditions described, rotary kilns with steel casings cannot be used since their stability decreases rapidly at temperatures above 1,000.degree. C. Further, ceramic linings can improve the abrasion resistance of the steel pipe at high temperatures and, under some circumstances, also reduce caking of material on the wall. However, such linings act as thermal insulators so that the indirect heating process cannot be used.